Friction stir welding is a solid-state joining technique that is well known to those of ordinary skill in the art. Typically, friction stir welding is used to join difficult-to-weld metals, metal alloys (such as aluminum alloys, titanium alloys, nickel alloys, and the like), and other materials. For example, certain aluminum alloys are sensitive in a plasticized heat-affected zone, where the base metal reaches temperatures between solidus and liquidus during welding. In this heat-affected zone, partial melting at grain boundaries forms a network containing brittle compounds. As a result, weld ductility is substantially reduced. Likewise, other conventional joining techniques may create geometric distortions near a weld joint due to high temperature gradients induced in a workpiece during welding. These geometric distortions may cause warping and other dimensional defects in the workpiece, as well as residual stresses that may cause premature failure by cracking in the heat-affected zone or weld joint, lamellar tearing, or by stress-corrosion cracking in some metals and metal alloys.
Friction stir welding techniques overcome many of the problems associated with other conventional joining techniques. In friction stir welding, a rotating, cylindrical, non-consumable pin tool is plunged into a rigidly clamped workpiece and traversed along the joint to be welded. The pin tool is specially designed to provide a combination of frictional heat and thermo-mechanical working to accomplish the weld. As the pin tool is traversed along the joint to be welded, the plasticized metal, metal alloy, or other material is transferred from the leading edge of the pin tool to the trailing edge of the pin tool, forming a strong solid-state weld joint in the wake of the pin tool. During the friction stir welding of hard metals, metal alloys, and other materials, relatively high temperatures are generated in the pin tool, as well as the tool holder. These relatively high temperatures in the pin tool, in combination with relatively low temperatures in the workpiece, may result in a weld joint of poor quality and mechanical integrity, prone to defects and workpiece distortions. For example, solid-state welds, including inertia welds, translational friction welds, and the like, associated with titanium alloys, such as Ti17 and the like, are typically characterized by poor fracture toughness and impact strength. This is due, in part, to the relatively high cooling rate of such welds using conventional joining techniques, including conventional friction stir welding techniques.
Thus, what is needed are thermal management systems and methods that allow a workpiece to be controllably heated during friction stir welding, such that the temperatures in the workpiece more closely match the temperatures in the pin tool. In this manner, the cooling rate of a weld joint may be controlled. This would result in a weld joint of enhanced quality and mechanical integrity, free from defects and workpiece distortions and demonstrating improved fracture toughness, impact strength, and fatigue properties. This would also allow for enhanced pin tool temperature control in the event that a consumable pin tool is used and minimize problems associated with pin tool debris entrapment. Finally, pin tool wear would be reduced and pin tool life increased.